This invention relates to cross-linked double base propellant compositions having improved low temperature strain properties at a high strain rate which propellants are suitable for use in tactical missiles.
Processes for preparation of cross-linked double base propellant are well known in the prior art. An illustrative prior art process for producing cross-linked double base propellant is described in U.S. Pat. No. 3,798,090 issued to John C. Allabashi, Mar. 19, 1974. This patent discloses the use of prepolymers of polyglycoladipate and tolylene diisocyanate as cross-linking agents for the nitrocellulose component of the propellant. This patent also discloses the use of certain prepolymers such as hydroxy-terminated polycaprolactones and aromatic and aliphatic diisocyanates as cross-linking agents for nitrocellulose.
Cross-linked double base XLDB propellants require good low temperature strain capability at very low temperatures in order to be of practical use in tactical missiles for such applications as air defense. In some rocket motors a propellant strain at maximum stress (e.sub.m) value of 6% or more is desirable at -65.degree. F. at a high strain rate (74 in./in./min/).
The low temperature strain capability of propellants is limited by the glass transition temperature of the propellant binder. The glass transition temperature of a propellant binder is dependent on the glass transition temperature and volume fractions of each of the binder components. In most XLDB propellants the binder consists of nitrocellulose, prepolymer such as polyglycol adipate, energetic plasticizer such as nitroglycerin, various stabilizers and a cross-linker.
The major ingredient of XLDB propellants is nitroglycerin which has a glass transition temperature -90.degree. F. The glass transition termperature of XLDB propellant is higher than that of nitroglycerin due mainly to the higher glass transition temperature properties for the polymers included in the binder, viz., +248.degree. F. for nitrocellulose and -38.degree. F. for polyglycoladipate. Various approaches to lowering XLDB propellant binder glass transition temperature that have had limited success include lowering the nitrocellulose content and increasing the nitroglycerin level of the propellant. These approaches, however, result in lower propellant strength due to the lower polymer content in the propellant.